Objective：To investigate the distribution and effect of  endothelial nitric oxide synthase (eNOS) on the zero-stress state of carotid arteries in rats at different ages.
Methods：Morphometry and microstructure of carotid artery in 3-month-old, 6-month-old and 12-month-old Sprague-Dawley rats were studied quantitatively by histological method and computer image analysis. Immunohistochemistry and the NADPH-diaphorase reaction were used to localize the endothelial nitric oxide synthas (eNOS) in different age rats.  The changes of opening angle in the zero-stress state was also observed.
Results：With increasing age, there was a significant reconstruction changes in morphometric parameters and microstructure parameters of the carotid arteries in rats. While the expression of eNOS protein of endothelium and vascular smooth muscle cells were decreased. Compared with 3-month-old rats, the zero-stress state. of opening angle in 6-month-old and 12-month-old group were increased. However, pre-incubation of vessel rings with L-arginine (30 mmol/L), a substrate of eNOS,  attenuated the changes of the zero-stress state.
Conclusions：The results suggest that the carotid arterial wall in different age rats shows unequal growth and different zero-stress state eNOS may be involved in the age-related changes of zero-stress state of the carotid artery in aged rats. The results may provid an experimental base for vascular transplantation and vascular reconstruction.